Unpopular? If I remember correctly, I counted yesterday and something like 20 character threads had fewer posts (most by several hundred) than the Lucas threads.
SWF is a pretty selective community, not exactly a place to look for random data sets. From what it seems with the 'play with anyone' mode, assuming that any loyal SWF member knows how to perform common advanced techniques and players that don't aren't members of SWF, I'd estimate that only about .5% of Brawl players are members of SWF.
Lucas just doesn't have that pizazz going for him that most unknowing first time players would be attracted to. Every non-gamer knows the Mario and Zelda cast, and most recreational gamers will have knowledge of the rest of the cast, with the exception of Japanese only releases, such as Fire Emblem and Mother. Even still, Marth and Roy were pretty flashy characters that attracted players, and Ness was in the only US Mother release, giving him a little credibility. Lucas doesn't have any of that going for him, so the average Lucas player probably picked him up due to prior knowledge of him or interest in more technical aspects of the game (exploits, glitches, uniqueness, etc.)
EDIT: Also, in my personal opinion, Lucas doesn't seem to have what it takes to cut anywhere above mid to high tier. Tier lists are based on common winning characters. In turn, characters that win more often do so because of a high knowledge and userbase. To complete the loop, many players tend to flock to the higher tiered characters. With a tier list for this game being developed everyday, it might be that many "pro" players have some sort of interest in Lucas, but they also know he probably won't be winning any of the major tournaments, thus using different characters to increase their chances.
Of course, there are certain people out there that strictly play uncommon characters, like Lucas may prove to be in the future, and are revered for it. That type of status, though, cannot be distinguished until after the facts that; Lucas isn't as good as half of the cast and this certain player does exceedingly well in tournaments with Lucas, moreso than other Lucas players; are already established, in which case we won't know for a good while who's decent with Lucas and who's great. Mind, this is all common sense and can be applied to any other character.
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I think Simna will probably be the best Lucas. After all, he's unearthly good with PK Thunder mindgames and reading opponents. I recall when I played him in a Melee tournament sometime last year he would never fail to kill me with PK Thunder 2 while edgeguarding.